The present invention relates generally to a field of sensors, and, more particularly, to designs and methods of assembly of tactile array sensors used for sensing pressure distribution. Specifically, the present invention describes the use of multiple capacitance-sensing integrated circuits (ICs) each controlling a limited number of individual capacitor sensors, these individual sensors together forming the entire tactile array. This arrangement allows for convenient and repeatable manufacturing of tactile array sensors, especially those with high number of closely located individual sensors.
Tactile sensing involves a continuous measuring of variable tactile force or pressure. In some respects, tactile sensing for electromechanical devices is analogous to the human sense of touch in that information about the amount and distribution of tactile pressure over a surface can be received and transmitted. Not surprisingly, tactile sensing finds great utility in the field of robotics where such tactile sensors provide signals for negative feedback control of servomechanisms and the like. Tactile sensing can provide information about shape, texture, position, orientation, deformation, center of mass, and presence of torque or slippage with respect to an object in contact with the sensor. Other applications of tactile sensing will come to mind to those skilled in the art.
The tactile sensor or tactile sensing transducers can be configured with an array of electrodes to provide a measure of the distribution of tactile pressure over a surface. Ideally, the tactile sensor will have sufficient sensitivity, consistent reproducibility, and high resolution.
One of the known methods employed in tactile sensing is the use of a medium whose electrical properties vary in response to pressure induced deformation. For example, some materials exhibit a piezoresistive effect, i.e. the electrical resistance of the material varies in response to its deformation. Layers of such material sandwiched between two conductive plates will provide a means for detecting pressure when an electrical potential is established between the two plates. The current flowing between them will therefore vary according to the deformation of the intermediate layer resulting from an external pressure forcing the plates closer together. The current will change according to Ohm's Law, such that measuring the current can provide a means for measuring the tactile force applied to the plates. Compression-sensitive materials currently in use include for example foamed polymers, which contain conductive fillers such as finely divided particles of metal or carbon. Polyurethane and silicone are also commonly used.
There are also known two-dimensional capacitive pressure sensors developed primarily for realizing in the so-called “touch pads” of portable PCs that allow the reconstruction of the position of an object weighing onto the surface of the sensor. U.S. Pat. No. 5,374,787 describes a sensor of the position of such an object onto a sensible surface. These devices are realized with manufacturing techniques of printed circuit boards (PCB), according to which a substrate of fiber-glass or of Mylar® is provided with copper orthogonal stripes defined on one or on the other face of the substrate. Notwithstanding that a substrate of Mylar® or of another dielectric material may be moderately flexible, at least for small deflections, the sensor so constructed remains substantially rigid and not pliable into different geometric shapes. It is evident that these known devices are unsuitable for covering multi-curvature shapes such as a robot fingertip, other organic shapes, or to be incorporated in any object that must retain flexibility and pliability to conform to different shapes as a fabric.
FIG. 1 illustrates the principle of creating a typical conductive cloth-based tactile sensor array. Such device generally consists of a top plurality of parallel electrodes 10 that are placed over the bottom plurality of electrodes 20 with a non-conductive elastic isolation layer therebetween. Both the top 10 and bottom 20 pluralities of electrodes can be made of a conductive cloth-based material such as LYCRA™ that can be stretched in one or both X and Y directions. Other materials such as conductive weave fabrics can also be used for this purpose. Individual electrodes can be made as metallized fibers, strands, or yarns that form such conductive cloth or in any other way that is known from the prior art. If soldering is to be used to connect electrodes to the wires of the control unit, the temperature stability of the fabric material should be chosen to allow soldering to take place. In a typical configuration shown on FIG. 1, top electrodes are positioned to be perpendicular to the bottom electrodes forming the intersection areas, which define individual pressure sensors.
The non-conductive material separating the two layers with electrodes is typically chosen to be elastic and compress under the force applied to it within the range of forces estimated to be the working range for each tactile sensor array. For each of the intersection areas in which the top electrode intersects the bottom electrode, a capacitor is therefore formed between the top electrode and the bottom electrode with a compressible non-conductive material therebetween. That capacitor is used as an individual pressure sensor. As the pressure of force is applied to each such sensor, the top electrode is moved closer to the bottom electrode with the compression of the non-conductive material separating the two electrodes. Voltage potential is applied to both the top electrode and the bottom electrode so the capacitance can be measured therebetween. Changing capacitance reflects the degree of pressure or force applied to each sensor in the array. Typically, one plurality of electrodes is designated as a Drive Strip and the other plurality is designated as a Sense Strip. Drive electronics can provide selective measuring of capacitance at any chosen point between these strips of electrical conductors. High speed scanning of all the points in the matrix results in a single data frame reflecting pressure distribution over the surface of the matrix.
FIG. 1 further illustrates a typical way to connect individual electrodes to the outside control unit using connections 11 and 21 for the first plurality of electrodes 10 and the second plurality 20 respectively. The total number of such connections is defined by the number of electrodes in the first plurality plus the number of electrodes in the second plurality. For large tactile arrays having closely positioned electrodes, reliable and consistent manufacturing of such connections presents a technological challenge.
Flexible tactile sensor arrays can provide useful information about pressure distribution along curved surfaces. Despite the great extent of knowledge developed in the prior art, practical use of tactile conductive cloth-based sensor arrays has been limited until the present time. This is caused by the difficulties in manufacturing the tactile sensor array with more than just a few electrodes. Once the number of electrodes exceeds about 8 on each side of the array (the total of 16), direct attachment of the control unit wires to cloth-based electrodes becomes a burdensome procedure. One great difficulty is managing the large number of wires on both sides of the tactile sensor array and connecting them repeatedly in a reliable manner without intermittent opens and shorts between wires or electrodes.
Another manufacturing difficulty associated with the prior art tactile sensors stems from the high pitch density sensor designs having electrodes located closely together, typically less than only about 3 mm apart. Attaching individual wires to such electrodes becomes difficult as conductive epoxy or solder exhibits a tendency during assembly to deploy over more than one interconnection pad and therefore short the electrical connections.
Our U.S. Pat. No. 7,430,925 (See FIG. 2) discloses one useful method of reducing the number of attachments and producing them reliably, namely by using jumper wires 12 or using a flex harness with electrodes 22 etched therein or printed with the conductive ink.
Recent advances in design and broad availability of capacitive-sensing IC that can measure 12 to 24 individual sensor elements with a single chip is known to be useful for applications such as mobile phones where only a limited number of sensing elements are required.
The present invention contains further improvements in the structure of the tactile sensor arrays and provides for designs aimed at easier and more reliable manufacturing of such sensors.